Akihiro Kikuchi

The development of high-current cables made of rare-earth barium copper oxide (REBCO) tapes is a key technology for high magnetic field accelerator magnets. A flexible round REBCO cable has been proposed as one of the candidates for the high-current cable. This REBCO cable consists of many tapes wound in multiple layers in a helical configuration on a metal core. The current transport properties of the cable at 4.2 K and in high magnetic fields are essential for the design of high magnetic field accelerator magnets. In this study, a solenoid-wound bobbin was developed to characterize the cable critical current (I_c) at 4.2 K and 18 T. Three 2-mm-wide REBCO tapes were prepared to fabricate the sample cables. The cable I_c was consistent with the sum of their total tape I_c at 77 K. The magnetic field dependence on critical currents of tapes and cables was successfully measured at 4.2 K. One of the three tapes had an additional copper plating on the REBCO layer side to improve strain performance and allow it to be wound on smaller cores. The cable current density was improved by using the additional copper-plated tape compared to the commercially available tape.

To improve the beam collision performance of SuperKEKB, installation of superconducting sextupole magnet in being considered. The main superconducting sextupole coils and three types of superconducting correction coils will be incorporated in this magnet. We are developing the correction coils using ultra-fine strand Nb₃Al superconducting cables with strand diameters less than 50 µm fabricated by the React & Wind method. In this paper, we report the measurement results of the superconducting performance of Nb₃Al cables of 0.6 to 0.8 m in length in the solenoid coils with radius of curvature from 15 mm to 25 mm.

In order to develop submerged pumps to transfer liquid hydrogen, high temperature superconducting (HTS) motors to drive them as one of key components are a promising candidate with low electromagnetic loss and large specific power density per unit mass. However, since armature windings wound using HTS wires have a couple of problems such as three-phase unbalanced currents and frequency limitations, HTS wires are used only for rotor windings of the motors and metallic cables have to be applied to their armature windings. Therefore, numerical analyses to evaluate loss properties in multi-strand metal twisted cables for armature windings of HTS motors cooled at liquid hydrogen temperature are carried out by means of a two-dimensional finite element method. On the basis of a clarified physical mechanism of losses generated in the windings arranged within a slot of armature iron core, the obtained numerical results are also reproduced with theoretical expressions of the Joule loss for an alternating transport current and the eddy-current loss for an externally applied AC magnetic field. The influences of losses on the frequency and number of strands in metal twisted cables are investigated quantitatively.

The Institute for Solid State Physics (ISSP), the University of Tokyo, has been developing non-destructive pulsed magnets for both fundamental and applied sciences in high magnetic fields. Currently, our high-field facility operates a series of capacitor-driven pulsed magnets for both user and in-house experiments. These systems are classified into three categories: (1) a short-pulse magnet up to 75 T with a 4 ms duration, (2) a mid-pulse magnet up to 60 T with a 36 ms duration, and (3) a long-pulse magnet up to 40 T with a duration exceeding 1 s. Recent work has been directed towards improving the performance of Cu–Ag alloy wires, enabling magnetic fields up to 88.6 T with a 2.5 ms duration using a mono-coil magnet. In addition, electric double-layer capacitors (EDLCs) with a total energy of 150 MJ have been installed for a long-pulse magnet, aiming to generate magnetic fields up to 60 T with a 1 s pulse duration.

An organic olefin-based thermosetting dicyclopentadiene (DCP) resin named TELENE was proposed as an impregnation material for Nb₃Sn magnets to reduce the training. To verify the radiation resistance of the TELENE, gamma-ray irradiation experiments and three-point bending tests were conducted on the TELENE and the CTD-101K. The TELENE mixed with Gd₂O₂S powder was also applied for these measurements. The gamma-ray irradiation experiment was carried out up to nearly 20 MGy at room temperature using a Cobalt-60 source at the Takasaki Advanced Radiation Research Institute in Japan. For gamma irradiation in the absence of air, all resin samples were sealed in glass tubes and evacuated. The flexural strength and flexural modulus of each resin were measured by the three-point bending test before and after gamma-ray irradiation. The pure and mixed TELENE exhibits plastic deformation before the irradiation and can withstand larger strain than the CTD-101K. After irradiation, no decrease in flexural strength was observed up to 6 MGy for the pure TELENE and up to 17 MGy for the mixed TELENE. However, their strains at fracture decreased by more than 50% after 5 MGy irradiation.

This paper reports a study of the low-temperature properties of 49-strand circular cables made of ultrafine National Institute for Materials Science MgB₂ wires. The critical currents of cables made from single-core MgB₂ wires with diameters of 0.050 mm and 0.022 mm and different sheath materials (Cu, CuNi, and Monel) were measured at temperatures from 4.2 to 35 K and external fields from 0 to 8 T. In addition, magnetization alternating current (AC) losses were measured at temperatures of 20–45 K, fields of 1–100 mT, and frequencies of 72 Hz and 144 Hz. The presented experimental results show high engineering current densities in spite of a very low filament size of 8.5 µm and also low AC losses due to well decoupled strands. We show that the total AC losses are not only affected by the filament size but also by the sheath materials used. The cables presented here can possibly be used for small windings with low AC losses.

The joint expertise of ANL and FNAL has led to the production of $\text{Nb}_{3}\text{Sn}$ undulator magnets in operation in the ANL Advanced Photon Source (APS). These magnets showed performance reproducibility close to the short sample limit, and a design field increase of 20% at 820 A. However, the long training did not allow obtaining the expected 50% increase of the on-axis magnetic field with respect to the ∼1 T produced at 450 A current in the ANL NbTi undulator. To address this, 10-pole long undulator prototypes were fabricated, and CTD-101K was replaced as impregnation material with TELENE, an organic olefin-based thermosetting dicyclopentadiene resin produced by RIMTEC Corporation, Japan. Training and magnet retraining after a thermal cycle were nearly eliminated, with only a couple of quenches needed before reaching short sample limit at over 1,100 A. TELENE will enable operation of $\text{Nb}_{3}\text{Sn}$ undulators much closer to their short sample limit, expanding the energy range and brightness intensity of light sources. TELENE is Co-60 gamma radiation resistant up to 7–8 MGy, and therefore already applicable to impregnate planar, helical and universal devices operating in lower radiation environments than high energy colliders.

We are approaching to high mechanical strength of bronze processed Nb₃Sn wire by internal matrix reinforcement using Cu-Sn-In ternary alloy. The In solute element remained homogeneously in the wire matrix after Nb₃Sn synthesis, and it contributed to transforming (Cu, In) solid solutions. And (Cu, In) solid solution acted as the protective material of Nb₃Sn phase and contributed to improving the mechanical strength of the Nb₃Sn wire. In this study, we investigated the Hot Isostatic Press (HIP) treatment on the further mechanical strength improvement of internal matrix reinforced Nb₃Sn wire. Tensile stress and strain sensitivities of transport critical current ($I_{c}$) property were enhanced by HIP processing because many micro-sized vacancies, which were the origin points of the fatigue on the (Cu, In) matrix, were removed. We found the HIP treatment contributes to mechanical strength enhancement of the internal matrix reinforced Nb₃Sn wire.